A solar eclipse occurs when the Moon passes between Earth and the Sun, thereby totally or partly obscuring the image of the sun for a viewer on Earth. A total solar eclipse occurs when the moon's apparent diameter is larger than the sun's, blocking all direct sunlight, turning day into darkness. Totality occurs in a narrow path across Earth's surface, with the partial solar eclipse visible over a surrounding region thousands of kilometers wide.

The last time a total solar eclipse was visible across the entire contiguous United States was during the June 8, 1918 eclipse, and not since the February 1979 eclipse has a total eclipse been visible from anywhere in the mainland United States.[3] The path of totality will touch 14 states (although a partial eclipse will be visible in all fifty states), [3] and 16% of the area of the United States.[4] The event will begin on the Oregon coast as a partial eclipse at 9:06 a.m. PDT on August 21, and will end later that day as a partial eclipse along the South Carolina coast at about 4:06 p.m. EDT.[3]

There are expected to be logistical issues with the influx of visitors, especially for smaller communities.[5][6] There have also been issues with counterfeit eclipse glasses being sold.[7][8][9]

Future total solar eclipses will cross the United States in April 2024 (12 states) and August 2045 (10 states), and annular solar eclipses — meaning the apparent size of the Moon is smaller than that of the Sun — will occur in October 2023 (9 states) and June 2048 (9 states).

This eclipse will be the first total solar eclipse visible from the United States since the solar eclipse of July 11, 1991[13] (which was seen only from part of Hawaii),[14] and the first visible from the contiguous United States since 1979.[15]

Some American scientists and interested amateurs seeking to experience a total eclipse participated in a four-day Atlantic Ocean cruise to view the solar eclipse of July 10, 1972 as it passed near Nova Scotia. Organizers of the cruise advertised in astronomical journals and in planetarium announcements emphasizing the lack of future total eclipses observable in U.S. until this 2017 event.[18]

The August 2017 eclipse will be the first with a path of totality crossing the Pacific and Atlantic coasts of the U.S. since 1918. Also, its path of totality makes landfall exclusively within the United States, making it the first such eclipse since the country's independence in 1776. (The path of totality of the eclipse of June 13, 1257, was the last to make landfall exclusively on lands currently part of the United States.[19])

The solar eclipse of August 12, 2045 will have a very similar path of totality over the U.S., about 400 km (250 mi) to the southwest, also crossing the Pacific and Atlantic coasts of the country; however, duration of totality will last over twice as long.[20]

An eclipse of comparable length (up to 3 minutes, 8 seconds, with the longest eclipse being 6 minutes and 54 seconds) occurred over the contiguous United States on March 7, 1970 along the southern portions of the Eastern Seaboard, from Florida to Virginia.[21]

During total eclipse the stars and 4 planets will be visible, including the star Regulus close to the Sun. Mars will be 8 degrees to the right, and Venus 34 degrees right. Mercury is 10 degrees left, and Jupiter 51 degrees left.

Craters of the Moon – The National Monument and Preserve will host NASA presentations, evening star parties hosted by the Idaho Falls Astronomical Society, high altitude balloon launches by the USC Astronautical Engineering department and NASA, and presentations by the New Mexico Chapter of the Charlie Bates Solar Astronomy Project.[28]

Kansas City, Missouri – A 5-mile bicycle ride from downtown KCMO (where totality will only last about 30 seconds) to Macken Park in North Kansas City (where totality will last 1 minute 13 seconds) has been organized by KC Pedal Party Club, a local Meetup group.[39]

Nashville, Tennessee – The largest city in the path of totality is offering many special events, including the Music City Eclipse Science & Technology Festival at the Adventure Science Center.[53] The Italian Lights Festival is hosting the largest Eclipse Viewing Party in Nashville, a free NASA-Certified Eclipse Event held at the Bicentennial Mall.[54] Two astrophysicists from NASA's Jet Propulsion Laboratory will emcee the countdown.[55]

In northwestern Europe, the eclipse will only be visible partially, in the evening or at sunset. Only those in Iceland, Ireland, Scotland and the Portuguese Azores archipelago will see the eclipse from beginning to end; in the rest of the UK, Norway, the Netherlands, Belgium, France, Spain and Portugal, sunset will occur before the end of the eclipse. In Germany, the beginning of the eclipse will be potentially visible just at sunset only in the extreme northwest of the country. In all regions east of the orange line in the map, the eclipse will be invisible.[62]

Counterfeit eclipse glasses have become a public health issue; deficient solar eclipse filters can quickly cause irreparable eye damage inasmuch as the retina has no pain receptors. In particular, it's not enough that the glasses have a logo and the ISO number 12312-2; customers must be reasonably confident that the glasses have come from a reputable manufacturer and/or authorized dealer. A person should not be able to see much through a safe solar filter except for the Sun itself, sunlight reflected off shiny metal, the hot filament of unfrosted incandescent light bulbs, bright halogen light bulbs, bright LED flashlights, and an arc-welder's torch (a welder's helmet can be used for safe viewing provided it has a filter of 12 or higher). [7][8][9]The American Astronomical Society, in their listing of reliable vendors of eclipse glasses, states that not only must effective glasses dim the visible light of the Sun to a comfortable level, they must also filter most ultraviolet and infrared light.[7]

Logistically, the demand for portable toilets will be "astronomical,"[65] with municipalities inside and alongside the path of totality planning (in some cases for three years, with the participation of several levels of governance, as with southern Illinois) for the sudden influx of people.[66] Volunteers in smaller towns (as with Lathrop, Missouri, which will be coincidentally celebrating 150th anniversary of its founding) struggled to arrange viewing sites and logistics for what could be either a tourism boom or a disaster.[5]

In the American West, illegal camping is a major concern, as with Jackson Hole, Wyoming.[6] Idaho's Office of Emergency Management noted that Idaho was a prime viewing state, and advised jurisdictions to prepare for increased loads on their services; nearly every hotel and motel room, campground, and in some cases backyards for nearly 100 miles (160 km) miles north and south of the path of totality had been reserved several months if not years in advance.[67] The state's population may temporarily increase by a third, with 370,000 to 500,000 visitors projected in state of 1.6 million people.[68]

Oregon Governor Kate Brown authorized the deployment of the National Guard (six aircraft and approximately 150 soldiers), given that the sudden influx of tourists will occur during the peak of that state's fire season; the soldiers will perforce also be available for fire-fighting duties if needed.[69] Furthermore, given that the expected increase in population of one million will closely mimic the increased demand in emergency services, hospital staffing will be augmented (overnight tents will be provided), as well as blood supplies and anti-snake bite antidote (which has a short shelf-life) along the totality line. The emergency room of the hospital of Madras, Oregon (population 6,700), for example, is anticipating a six-fold increase in patients; as the hospital only has 25 beds, medical evacuation is anticipated.[citation needed] The closest precedent for statewide planners is the annual Sturgis motorcycle rally in South Dakota, when approximately half a million people descend on the town.[70]

In Oregon at one point on August 17, four days before the eclipse itself, traffic was backed up for fifteen miles on U.S. Highway 26 near Prineville, the turnoff to an eclipse festival, as thousands arrived; some gas stations there and in Bend, Oregon had briefly run out of product the day before.[71]

The eclipse will cause a reduction of solar power where the shadow reaches solar panels. The North American Electric Reliability Corporation measures impacts of this event,[72] and predicts minor impacts.[73] In California, solar power may decrease by 4–6,000 Megawatt[74] at 70 MW/minute, and then ramp up by 90 MW/minute as the shadow passes. CAISO's typical ramp rate is 29 megawatts per minute.[75] Around 4 GW mainly in North Carolina and Georgia are expected to be 90% obscured.[74] The Solar eclipse of March 20, 2015 caused manageable solar power decreases in Europe;[76] in Germany, solar power dropped from 14 GW to 7 GW, of a 38 GW solar power capacity.[77]

This solar eclipse is a part of Saros cycle 145, repeating every 18 years, 11 days, containing 77 events. The series started with partial solar eclipse on January 4, 1639, and reached a first annular eclipse on June 6, 1891. It was a hybrid event on June 17, 1909, and total eclipses from June 29, 1927 through September 9, 2648. The series ends at member 77 as a partial eclipse on April 17, 3009. The longest eclipse will occur on June 25, 2522, with a maximum duration of totality of 7 minutes, 12 seconds. [80]

The metonic series repeats eclipses every 19 years (6939.69 days), lasting about 5 cycles. Eclipses occur in nearly the same calendar date. In addition the octon subseries repeats 1/5 of that or every 3.8 years (1387.94 days).

21 eclipse events, progressing from north to south between June 10, 1964 and August 21, 2036

^"Thousands Go West for a Total Solar Eclipse Tomorrow; Data May Aid Energy Research Partial Eclipse for New York Best Types of Film Image of Sun on Screen", The New York Times February 25, 1979. p. 26.

^"2016 Long-Term Reliability Assessment"(PDF). North American Electric Reliability Corporation. December 2016. p. 70. Retrieved April 18, 2017. causes substantial effects to wide-scale solar generation within a very short amount of time. The output generated by PV/solar systems will be either diminished or drastically reduced within the window of this event. Sudden widespread diminishing of solar irradiance may heavily affect areas with large amounts of utility scale PV energy installations or behind-the-meter DERs.